SLIM JIM
ANTENNA
PROJECTSeveral designs rolled into one Edited and
condensed from various designsPage updated with new information and
videosThe Slim Jim
AntennaThe Slim Jim is a vertically polarized
omnidirectional end-fed antenna having considerable "gain" and this
is concentrated almost parallel to ground toward the horizon rather
than skyward making it more efficient than a ground plane type
antenna by about 50 percent better. It can be built for almost any
frequency! ( Below 10 meters it gets VERY tall )Due to it's
SLIM design, there is very little wind loading.It is fed with 50
ohm coax.

It uses a 'J ' type matching stub (J Integrated
Matching = JIM), hense the name SLIM JIM. Credit for the original
design goes to F.C. Judd, G2BCX. Since the vertical angle of
radiation is so narrow, about 8 degrees toward the horizon, it
usually out performs 5/8 wave or groundplane type construction due
to their much higher angle of radiation. It is estimated that the
Slim Jim appears to have about 6dB gain over a 5/8 wave
antenna due to the extreme low angle of radiation.

(Most of the
radiation is directed toward the horizion making the "gain" appear
much greater than other vertical type antennas it has been compared
to with A/B testing)Editor's
note:There are many gain figures
quoted for this antenna and also various descriptions of the actual
type of antenna on various websites.Some have
even stated that, "In fact I found it outperformed a 1/2wave over
1/2wave over 1/2wave colinear!" No matter what you call it, it
seems to do an excellent job according to most reports. What have
you got to loose? Please let us know your results.....email us!
n4ujw AT hamuniverse.comUsing heavy duty construction would make this
a good omni repeater antenna. When correctly matched for lowest swr,
it has wide bandwidth.

Drawing on right shown with antenna mounted on PVC
pipe

Construction
details:NOTE: NO PART OF THIS ANTENNA SHOULD BE
GROUNDED!It should be totally
insulated from it's mount, mast, tower, etc with at least 1/4
wavelength of "freespace" distance. Formulas are provided below for
all the measurements including the freespace distance.The Slim
Jim should be constructed from 1/2" copper pipe. Also old tv antenna
elements or aluminum tubing could be used with some ingenuity and
would be lighter. Experimentation with heavy guage wire supported
inside PVC tubing or attached to insulated material such as wood
could also be tried and would probally be successful with some
ingenuity. 300 ohm twinlead
versions also work great!

Using copper pipe, bends
are made with soldered 90 degree copper elbows. An adjustable slip
sleave made from copper can be added to the element on top above the
gap for tuning purposes or possibly some sort of nut, bolt
arrangement soldered into the upper end to adjust spacing if needed.
(See the 2 meter SSB loop
project on this site for better details and pictures of
the nut, bolt arrangement.) Depending on the frequency or
band, the average length of the gap and spacing between the elements
is 3" at 72MHz and 1" at 220MHz. (See updates
below)
For 2 meter
work this would be around 1 1/2 to 2 inches. Some experimenters
report about 1 inch or less works well. Experiment with the
adjustment for best results. The recommended mount is the use
of PVC pipe and PVC pipe "T's."

Testing and tuneup:Support the antenna as high as possible from the
ground and other nearby objects especially metal, and fit the
coaxial cable to the antenna with some crocodile (alligator) clips.
It is suggested that the center conductor be attached to the longest
element, shield to the shortest. See diagram above. Attach about 2
to 4 inches up from the bottom and check the VSWR at the design
frequency. USE LOW POWER!Adjust the clips up or down to get the best
match, mark where they are to be finally installed, remove the
clips, and solder the coax directly or use clamps, screws, etc.
Waterproof or seal all connections and the end of the coax. Use the
copper sleeve or nut bolt arrangement, if added, for any necessary
tuning.

FORMULAS(For results in
inches)

NOTE: Air gap and
element spacing may have to be determined by some
experimentation for various frequencies. See new info about gap
spacing below.

10 Meters
28.400mhz8415 / 28.4mhz =
296.30 inches (24.69 feet)5610 / 28.4 = 197.5 inches
(16.45 feet)2805 / 28.4 = 98.76 inches (8.23 feet)Freespace
mounting distance 103.97 inches (8.66
feet)17
Meters!A 52 foot vertical including minimun distance
from ground!Hay don't laugh! It might be worth a try for about 6
db more!Please send us your input if you have suggestions for
any band using this antenna!The lengths will
have to be adjusted slightly for the addition of the top and bottom
connection points.See Construction and Testing tips
below.

CONSTRUCTION and TESTING
TIPS=============The Slim Jim should be constructed
from 1/2" copper pipe OR near this size of any conductive material
but this is not an absolute! The bends are made with soldered 90
degree copper elbows if your using copper tubing. A slip
sleave or other arrangement can be added to the upper or
lower part of the gap made from copper, brass or aluminum for
adjustment of the gap measurement for swr tuning, although the
average length of the gap and spacing between the elements is 3" at
72MHz and 1" at 220MHz. Some experimentation may be needed for gap
distance.

For 2 meters, this would be about 1 1/2 to 2
inches. Here again, this measurement is not extremly critical and
the gap, element spacing and element length all interact.The
total distance from the top of the gap around the entire length and
back to the bottom of the gap should equal about 1.5 wavelengths or
in the case of the 2 meter example above about 115.26 inches.No
part of the antenna should be grounded to the tower or mast.
Therecommended mount is the use of PVC pipe and PVC pipe
"T's." Make sure the space between the tower or mast and
the antenna is one "freespace" 1/4 wavelength.
TESTING:========Stand upright (on a
railing or non-conductive object, clear of metal surfaces, drain
pipes, etc.) and fit the coaxial cable to the antenna with some
crocodile (alligator) clips. Attach about 2 to 4 inches up
from the bottom (at 2 meters). It is suggested that the center
conductor be attached to the longest element, shield to the
shortest and using just enough power to get an swr reading
check the VSWR. Adjust the clips up or down to get the best match,
mark where they are attached, remove the clips, and solder the coax
directly. Seal connections and end of coax! Use the copper
sleeve, or other spacing adjustment if added, for any necessary
tuning. You may not get that perfect
1:1! The air gap,
total length and element spacing all
interact.

RECENT INPUT FROM SOME
BUILDERS:12 - 03 - 06Dear Om

Recently I returned
back to this City of Visakhapatanm, AP,INDIA.

I was looking for a
simple omni-directional antenna. Then happend to see ur slimjim
design and tech details.

Immediately brewed one
as follows:

Freq: 145 Mhz (
being our center freq. in VHF )

PVC Pipe
support: 32 MM Dia.

Ant. Eliment : 15 SWG
Copper Enamilled.

Gaps and spacings used
: 152, 98.3 , 49.2 CM

Gaps:
1.1/2" Feed Point: 2" Gr . Clearance: 50
CM.

Performance:
Excellent ( The stations which were very feeble , with GP now are
very strong , and even 0.5 W is enough for me . ( we have
no repeater )

I have No SWR meter,
and simply depended on your design.

So many thanks to u
for the support to ham community.

The following hams (
brewed along with me ) conveyed their thanks to
u VU2DFB, VU3KVF, VU2NDJ .

Just to report that yesterday I built the
Slim Jim for 2m using the instructions I found in your site. Without
any changes to your design, it made an increase of 4 S points
(measured in my FT-857D screen) the reception of a repeater 60 km
away.

4 S points in an FT-857D is to be taken with caution.
It only shows a *qualitative* increase in reception. I am
afraid, S points in my transceiver cannot be related to
quantitative measures.

(Note: Normal calibrated S meters yield
about 6db per S unit.....there is NO way this antenna has 24db gain.
Dimitri compared the Slim Jim to a 1/4 wave ground plane.) (See
below for more). It does seem to be a very good antenna for 2
meters or any other band where the physical size would not be a
mounting problem.

I
agree!

What were you using before the Slim Jim to compare it to and
how high was the old antenna and how high is the new Slim
Jim?I used a home-built 1/4 lambda ground plane,
built using the ARRL Antenna Handbook instructions. Photos can be
seen in http://f4dyt.free.fr

Height is of relative value, since
the Ground Plane had a... ground. And Slim Jim is very insensitive
to height. Both antennas were installed as indoor VHF antennas at
the attic of my house, both at around 3 meters above ground. It was
an A/B test.

I did a QRP test with 5 watts using a repeater
24 km East from home. I don't have a clear view, so some
diffraction may affect results (if I am in a reinforcement zone,
for example). My correspondent gave me a 57 with very little noise,
and a very good signal strength for a QRP station.

I would
like to report that I modeled the antenna on MMANA (I also had to
convert all units into metric ones). My simulations suggest that the
theoretical height would have been 1.73 m, and that at 1.47 m
the antenna should NOT work. Of course, reality won ;-)

I also want to report that the firing angle is very low. For
example, before;I was able to listen to airplanes going to the
Charles de Gaulle airport at 121.500 MHz (AM) which are flying over
our heads (we are below an aerial corridor). Now I listen to them
much weaker, while the airport tower enters at S9++ (again, FT-857D
measures not to be trusted at all). The MMANA simulations also
suggest a maximum gain between 3 and 4 degrees, with a secondary
lobe at 60 degrees.(Dimitri's MMANA simulations confirm the very low
angle of radiation that this antenna is noted for. Several
months testing at the N4UJW QTH using this antenna to listen to 2
meter transmissions from the station aboard the International Space
Station also comfirms the extreme low angle of the pattern. It has
been noted that on all passes over this QTH, the 2 meter signals
were MUCH stronger at and below 8 degrees above the horizon with the
signal strength decreasing as the ISS approached
overhead!........N4UJW)73 Dimitri F4DYT -- Dimitri Aguero -
daguero@free.fr F4DYT - Locator JN18bv
Saint-Germain-en-Laye, France

I perused your webpage - I'm a condo ham and can't
have any outdoor antennas. Also don't like the thought of a
G.P. (with it's radials) hanging from the ceiling. Went to the
local hardware emporium and got some wire and a legnth of PVC
double-wall pipe, and 1 bar stool "foot" for each end. Drilled
a hole at the top....used number 16 wire for the elements , held it
all together with electrical tape. Put it in the corner of the
"shack" (a bedroom ) fired 5W from my ancient IC228H rig....and it
works like a champ. Getting good reports from 5W and an indoor
slim jim. It is a great apartment antenna. Take care
thanks 73

Greg
WB9MII

Illinois

Dual Band
Slim Jim Video from PU2OKE....see more from him on YouTube(Turn
your sound up!)